1. Field of the Invention
The invention relates to a knock control apparatus/method for an internal combustion engine which determines that knocking has occurred if the intensity of engine oscillation detected during a predetermined knock determination period is equal to or higher than a predetermined knock determination level and which performs retardation control of ignition timing on the basis of a result of the determination.
2. Description of the Related Art
In an apparatus of this kind, as a rule, a knock determination period for detecting engine oscillation resulting from combustion of a mixture is determined in advance, and a peak value of the engine oscillation is detected while the engine oscillation is monitored during the knock determination period. If this peak value is larger than a preset knock determination level, it is determined that knocking has occurred. In addition, if it is thus determined that knocking has occurred, an ignition timing is generally controlled so as to be retarded.
There is also known an apparatus that corrects a knock determination period in accordance with an engine operational state so as to make the knock determination period coincident with a knocking occurrence period.
For an internal combustion engine equipped with a variable valve timing mechanism that variably sets timings for opening and closing engine valves in accordance with an engine operational state, there is also available an apparatus that corrects a knock determination period in accordance with timings for opening and closing the engine valves and that prevents engine oscillation from being caused by the sitting of the valves during the knock determination period.
In a reciprocating engine, because of its structure, pistons disposed therein change in posture while making reciprocal movements. As a result, engine oscillation may be caused.
A mechanism by which this engine oscillation is caused will be described hereinafter with reference to FIG. 9. First of all in a compression stroke, as shown in FIG. 9(a), a connecting rod 101 presses a piston 102 upwards with respect to an engine from a diagonally low region (from a diagonally left-low region in FIG. 9(a)). At this moment, while being pressed upwards with respect to the engine, the piston 102 is pressed onto an inner wall of a cylinder 103 by a force acting in a radial direction of the cylinder (to the right in FIG. 9(a)) indicated by an arrow A in FIG. 9(a), as a result of inclination of the connecting rod 101.
Then, as the piston 102 approaches a compression top dead center (a position shown in FIG. 9(b)), a high combustion pressure is applied to a piston ring 104. Thus, the piston ring 104 is pressed hard against the piston 102. A frictional force applied to a sliding face between the piston ring 104 and the piston 102 inhibits the piston 102 from being displaced in the radial direction of the cylinder.
Then, upon a shift to an expansion stroke, the piston 102 is pressed downwards by a combustion pressure. At this moment, as shown in FIG. 9(c), the direction of inclination of the connecting rod 101 is opposite to the direction of inclination thereof in the aforementioned compression stroke. Hence, as a result of this inclination, a force acting in the radial direction of the cylinder (to the left in FIG. 9(c)) indicated by an arrow B in FIG. 9(c) is applied to the piston 102. At this moment, since the piston ring 104 inhibits an upper portion of the piston 102 from being displaced in the radial direction of the cylinder, the upper portion of the piston 102 remains still. Only a lower portion of the piston 102, which is not inhibited from being displaced, is pressed onto the inner wall of the cylinder 103.
Then in an intermediate period of the expansion stroke, as shown in FIG. 9(d), the pressure of combustion decreases as the piston 102 moves downwards with respect to the engine. This leads to a gradual decrease in a binding force exerted by the piston ring 104 to inhibit the piston 102 from being displaced in the radial direction of the cylinder. On the other hand, the angle of inclination of the connecting rod 101 gradually increases, which also leads to a gradual increase in the force applied to the piston 102 in the radial direction of the cylinder as indicated by an arrow C in FIG. 9(d). As soon as this force exceeds the binding force exerted by the piston ring 104, the upper portion of the piston 102 instantaneously rotates from a state indicated by alternate long and short dash lines in FIG. 9(d) to a state indicated by solid lines in FIG. 9(d), around a point (point D) that is in contact with the inner wall of the cylinder 103. Hereinafter, the rocking of the piston thus caused in an intermediate period of an expansion stroke will be referred to as “swinging movements”. Because of the swinging movements, the piston 102 collides with the inner wall of the cylinder 103. At this moment, engine oscillation occurs.
As described hitherto in detail, the swinging movements are caused while the pressure of combustion falls in an expansion stroke, namely, in the vicinity of an end timing of the aforementioned knock determination period. Hence, if the magnitude of engine oscillation resulting from the swinging movements increases, it may be erroneously determined that knocking has occurred, despite the absence of knocking.